start with the best condition micra shell you could find (if ur bodyshells ok carry on, if not find a good one), strip the exterior down to the shell, treat all rust, divert the flawed sunroof drain pipe that causes all micra sills to rot, undercoat every nook n cranny of the entire underbody thats exposed, reassemble/swap parts from bad shell to good shell.
tis a long thorough process but is what i did to mine to prolong its life. check me blog for details